This specification relates to membranes, particularly ion exchange membranes.
Ion exchange membranes may be used to treat and remove ionizable components from fluids for a variety of applications. The ion exchange functionality operates to transport one type of ion across an ion exchange polymer in an electric field, while substantially or effectively blocking most ions of the opposite polarity. For example, anion exchange polymers carry cationic groups, which repel cations and are selective to anions.
Anion exchange polymers may be prepared from tertiary amines, which are quaternized to provide anionic functionality. The quaternary ammonium compounds are crosslinked and polymerized to form anion exchange polymers. Typical methods for making anion exchange polymers require the use of alkyl halides for quaternizing the anion exchange polymer.
U.S. Pat. No. 7,968,663 provides another example of an anion exchange polymer that can be used to make a membrane. The polymer is made by reacting a tertiary amine, an acid inhibitor and a polyepoxide to form a quaternary ammonium monomer in the presence of a catalyst. The exchange polymer is prepared without using alkyl halides.
An ion exchange membrane is made by embedding a membrane dope in a stable reinforcing fabric. The reinforcing fabric is typically made from a textile polymer such as polypropylene, polyester, polyvinyl chloride or polyethylene. The dope may comprise an ionic monomer containing a quaternary ammonium group (anion exchange) or a sulfonate group (cation exchange) along with a crosslinking co-monomer containing polymerizable diacrylic functionalities. The reinforcing fabric determines the thickness of the membrane. The fabric is typically pre-treated to improve its wettability and compatibility with the ion-exchange polymer but the pre-treatment may introduce contaminants into a membrane.